Contact damage modes in cyclic loading with spheres areinvestigated in three nominally brittle ceramics, soda-lime glass, porcelain andfine-grain silicon nitride, in moist environments. Initial damage at small numbersof cycles and low loads consists of tensile-driven macroscopic cone cracks("rittle" mode). Secondary damage at large numbers of cycles and high loadsconsists of shear-driven distributed micro-damage ("quasi-plastic" mode), withattendant radial cracks and a new form of deeply penetrating subsidiary conecracks. Strength tests on indented specimens are used to quantify the degree ofdamage. Both damage modes degrade the strength: the first, immediately aftercone crack initiation, relatively slowly, the second, after development of radialcracks, much more rapidly. A fracture mechanics model describing the first mode,based on time-integration of slow growth of cone cracks, is presented. Thismodel provides simple power-law relations for the remaining strength in termsof number of cycles and contact load for materials design. Extrapolations ofthese relations into the quasi-plastic region are shown to be non-conservative,highlighting the need for further understanding of the deleterious quasi-plasticmode in tougher ceramics. Comparison with static contact data indicates a strongmechanical (as opposed to chemical) component in the cyclic fatigue in thequasi-plastic region.
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